By Rob Mitchum
Us land animals like to think we’re so special. For instance, our spines are typically organized into five regions — cervical, thoracic, lumbar, sacral, and caudal — each with their own distinct vertebral anatomy. Because aquatic species often have much simpler spinal morphology, usually split into mere body and tail segments, paleontologists often use the spines of fossils to determine whether ancient, extinct species were land or sea dwellers. But a happenstance fossil find by a University of Chicago graduate student questions that assumption, and raises new questions about the evolution of features once thought to be the exclusive domain of walkers.
Lauren Sallan wasn’t looking to rewrite these spinal rules when she examined a fossil specimen of Tarrasius problematicus, a 350-million-year-old eel-like fish, kept at the National Museums Scotland in Edinburgh. Instead, Sallan was trying to assign Tarrasius to its proper place in the evolutionary tree of ray-finned fishes, today the dominant family of aquatic species. But when Sallan, a graduate student in the Program in Integrative Biology at the University of Chicago Biological Sciences, looked at the NMS Tarrasius fossil, she found an anatomical feature that, according to conventional wisdom, should have only been present in the land-dwelling tetrapods.
“I shouted out ‘why does a fish need a sacrum?’,” Sallan said. “It doesn’t have any pelvic fins, and yet it was so obvious that it has this heavily reinforced section right in the same spot as it would be on a tetrapod.”
A fuller characterization of the Tarrasius spine, published yesterday in Proceedings of the Royal Society B, determined that its anatomy could be split into five distinct sections, each separated by an abrupt boundary. When Sallan went back and looked at other specimens of the species, she found similar morphology that had been overlooked by other scientists who had studied the species, perhaps due to availability and bias.
“This was in a different museum from where most of the specimens are, and previous workers had just been looking at the same fossils over and over,” Sallan said. “It’s basically an issue of finding what you expect to be there instead of what’s actually there.”
In evolutionary terms, the discovery of a tetrapod-like spine in a decidedly un-tetrapod-like species takes away yet another characteristic once thought to be unique to land animals. Just last year, University of Chicago researchers studying the homely lungfish revealed that walking behavior wasn’t beyond the abilities of fish, even those with spindly tentacles instead of limbs. The presence of a multiple vertebral segments in an eel-like fish that likely wasn’t even showing that sort of primitive locomotion means that this spinal organization also does not signify walking ability.
“It’s the last trait to fall,” Sallan said. “First, limbs were thought to show that a species was on land and walking, and now the vertebral morphology doesn’t mean that they’re on land either. So a lot of the things we associate with tetrapods actually arose first in fishes, and this is another example of that.”
While it lived quite a long time ago, Tarrasius problematicus is still more recent than early tetrapod fossils such as Tiktaalik that are thought to represent the first land creatures. It also appeared long after the ray-finned fishes and lobe-finned fishes (which eventually begat tetrapods and, much later, humans) split from each other. So Tarrasius is not an ancestor of the first land animals, but rather an example of “convergent evolution,” where a particular feature arises more than once over the course of evolution. Tarrasius may owe its complex spine to inhabiting similar conditions to the early tetrapods: shallow “near-shore” environments such as reefs or flood plains. But whereas the five-segment spine of the early tetrapods went on to form the literal backbone of everything form reptiles to birds to mammals, the anamalous spine of Tarrasius was likely a “dead end,” Sallan wrote.
“Terrasisus seems to be a one off. After that one site where it’s found, you don’t seem to see it again in the record,” Sallan said. “There just seems to be a drive to have that morphology there, and then it’s gone.”
The unexpected spinal similarities also raise questions about a pillar of developmental biology: the Hox patterning system. In the 1980’s, scientists discovered gene sequences that direct body patterning in flies, and similar sequences and developmental programs were soon discovered in other vertebrate species. Fish have Hox genes too, but are thought to express them in different patterns from land animals, patterns that wouldn’t be expected to produce the five-segmented spine of the tetrapods. So the appearance of such a spine in Tarrasius questions whether the Hox genes work the same way in both vertebrates and fishes, or are even truly essential for the development of these spinal segments. Since most knowledge about the relationship between Hox genes and body pattern development is based on genetic studies of modern tetrapods, the new finding emphasizes the need for more testing in fish species.
“Part of the problem is that the Hox expression data is available only for a few model organisms,” Sallan said. “What’s really needed is some expression data from other ray-finned fishes and tetrapods, things that are not mouse and chick and zebrafish. We need to try to get the full diversity of Hox expression.”
A final mystery is how Tarrasius employed its unexpectedly intricate spinal column in its daily life. Sallan speculates that the bony vertebrae may have been useful in propelling the fish’s body during fast swimming, similar to the stiff vertebrae that modern marlins possess.
“I think it must help with stiffening the body, because the tail is so flexible,” Sallan said. “If you look at the general shape, it’s more like a tadpole or an early tetrapod, so it might just function to hold the body steady because the tail is flapping.”